The present invention relates to a drilling tool and more particularly to a composite sintered abrasive twist drill for drilling holes in printed circuit boards.
In the manufacture of circuit boards patterns of electrically conductive foil are imprinted on boards made of fiberglass-reinforced plastic. Since the patterns are formed on both surfaces of a double-layer board, plus in the interior of multi-layer boards, electrical connection between conductors of adjacent patterns is accomplished through metal plating. Small holes are drilled in the board through the conductors and a conductive metal then deposited from one conductor through the hole to the other conductor or conductors. To ensure proper electrical contact with all of the conductors the metal must flow through the plate or coat the entire surface of the hole. Therefore, each hole must be regular in size, have a smooth surface, and be free from burrs and resin smear. This requires a very sharp drill and precise drilling techniques.
The printed circuit boards are usually made of copper clad glass-epoxy laminates which are extremely abrasive. Therefore, a drill which is suitable for use with such materials should be as wear resistant as possible in order to maintain a cutting edge. For efficiency in manufacture similar boards are commonly stacked and drilled simultaneously. The drills must, therefore, be of sufficient length to pass through three or more boards, and for acceptable top to bottom registration, they must be rigid. Because of the small size of the holes, i.e., 0.006 to 0.125 inches, the drills are very slender and elongated. This fact, coupled with the axial thrust loading applied to the drills, dictates that the drill material should be as tough as possible. These properties, i.e., rigidity, toughness and resistance to wear are mutually exclusive, since the most wear resistant materials tend also to be brittle, while the toughest materials tend also to wear rapidly. These differing requirements have therefore necessitated comprises in the choice of the materials heretofore used in the manufacture of such drills. While drills have been made of various materials, the composition of materials used in the body of each drill has been uniform over its entire length. As a result, the cutting surfaces have been somewhat less wear resistant and the remainder of the body somewhat more brittle than would otherwise be desireable. Tool designers have thus found great challenge in selecting tool materials with the best compromises between wear resistance and toughness. Beyond a certain point marginal improvement in wear resistance has substantially increased the risk of an unacceptable rate of tool failure from breakage; conversely, an unbreakable tool has proven to wear very rapidly.
In the drilling process rapid removal of the chips is necessary to prevent overheating of the drill which causes dulling of the cutting edges and also melting of the board material, which in turn produces resin smear in the hole. Therefore, a twist drill having helical flutes is commonly used between 750 and 1500 times in drilling multi-layer boards and then resharpened to restore the cutting edges. Because of the back taper given to the drills they can usually be resharpened only three to five times before the diameter, and corresponding, the diameter of the hole, is reduced and becomes unacceptable. Heretofore the most wear resistant materials known, i.e., diamond and cubic boron nitride, have not been available for use in drills of the size employed with printed circuit boards. The difficulties involved in handling crystalline compacts of diamond or cubic boron nitride of the very small dimensions required for abrasive tipped printed circuit board (PCB) drills, and then the further difficulties of grinding such compacts to produce the webs, lips, and edges, have precluded the use of these materials in drills of the sizes required.